The relationship between species diversity and their distance from the low tide mark at Maungauika/North Head Historic Reserve, New Zealand
The relationship between species diversity and their distance from the low tide mark at Maungauika/North Head Historic Reserve, New Zealand
Hypothesis
Based on the hypothesis that species diversity increases with proximity to the low tide mark, we can suggest that the distribution of different species in a given area rises when moving down-shore. It is plausible that unique environmental conditions near the low tide mark, such as fluctuating water levels, food availability, and air/sun exposure, may create a more diverse range of habitats that support a greater variety of species.
North Head Field Trip
On the 18th of March, 2023, my class took a day trip to Maungauika/North Head Historic Reserve to explore the patterns of species diversity, reviewing 81 species inhabiting the intertidal region and their respective zones. There are four habitat zones: periwinkle, barnacle, oyster, and hormosira (Fig. 1).
Upon placing a ¼ meter quadrat 27 times at random within each zone, present species were calculated based on mobile individuals and encrusting species. While I could not attend the scheduled field trip, I decided to visit North Head alone at low tide to gather an understanding of the collected data and the surrounding environment. To support the hypothesis, I selected 15 species across each zone to study.
Results
The periwinkle zone is the furthest from the low tide mark and has the lowest diversity, with ten species covering this area (Fig. 1). The most abundant species calculated was Austrolittorina sp (Nodolittorina antipodum), with 139 individuals per m² (Fig. 2a). Nerita melanotragus was also quite prevalent, with 40 per m², significantly increasing in abundance within the barnacle zone, with 72 per m².
Three of the four encrusting species covered less than 0.2% of their quadrats, with Chamaesipho columna, Hormosira banksii, and Xanthoria parietina showing presence while Corallina officinalis were absent. The abundance of species is lower in the barnacle zone compared to the periwinkle zone; however, it is apparent that diversity increases, with 22 species inhabiting this area (Fig. 1). An increase in the encrusting species begins to show in this zone (Fig. 2b). C. columna covered 10% of the quadrat, while C. officinalis covered 0.2%. These were the only encrusting species present. Midge larvae only populated the barnacle zone with 12 per m² and were not recorded in any other area.The oyster zone shows increased abundance and diversity, with 30 species populating this area (Fig. 1). However, there was a significant decrease in N. melanotragus compared to the barnacle zone, lowering to 4 per m² (Fig. 2c). A large increase in Diloma bicanaliculata is shown, with 24 per m². An abundance and diversity increase in the encrusting species was evident, with the presence of each of the four selected. C. columna increased coverage from 10% to 16% compared to the barnacle zone, while the other encrusting species averaged 0.9%. The hormosira zone homed 35 species, displaying yet another increase in diversity (Fig. 1). There was a massive escalation in the abundance of Hormosira banksii and Petrolisthes elongatus when compared to the oyster zone, with 63% coverage (H. banksii) and 65 per m² (P. elongatus) (Fig. 2d). Likewise, C. officinalis greatly increased to 14%, while N. melanotragus was absent. Lunella smaragdus was the only species to appear within each zone, with peaking abundance in the oyster zone at 26 per m².When tested for the rank correlation, there was a perfect positive relationship between the zone’s proximity to the low tide mark and the species within that zone [R=1]. The t-test resulted in the value being infinitely large, indicating strong evidence for the hypothesis.
Fig. 1. The number of species present across each of the observed locations on North Head’s shore.
Fig. 2. The abundance of the 15 selected species within each of the four zones of North Head’s shore, shown in both number of individuals and percentage covered per m² (multiplied by 4, data collected using ¼ m quadrat) a) Periwinkle zone, b) Barnacle zone, c) Oyster zone, d) Hormosira zoneDiscussion
Discussion
These results provide a detailed analysis of the distribution and abundance of species across four intertidal zones, ranging from the periwinkle zone to the hormosira zone. One of the significant findings of this study is that the species diversity increases as we move closer to the low tide mark, supported by the rank correlation test results, indicating a perfect positive relationship between the zone’s proximity to the low tide mark and the species within that zone.
However, the study also reveals some interesting trends in species abundance within each zone. For instance, the furthest from the low tide mark, the periwinkle zone has the lowest diversity, with Austrolittorina sp (Nodolittorina antipodum) being the most abundant. This species is limited to the periwinkle zone because rock attachment is their key to survival; therefore, they inhabit higher shore areas to avoid constant wave action that could lead to dislodgement (O’Dwyer, Lynch & Paulin, 2014). N. melanotragus is also present, though abundance significantly increases within the barnacle zone; This is because of their flourishing nature within semi-sheltered rock crevices (Staveley Parker, 1976), which were more prevalent mid-shore, hence their absence in the hormosira zone. While L. smaragdus was found in all zones, though most prevalent in the oyster zone, this species migrates up-shore during high tide and down-shore during low tide (Alfaro, 2006), being most abundant in the hormosira zone during the field study as they migrated to feed on brown algae such as H. banksii. Likewise, P. elongatus takes cover under the rocky substrate in North Head’s lower tidal zone for shelter and feeding (Dahlstroml, Campbell & Hewitt, 2012).The dispersal patterns of D. bicanaliculata are similar, but the determining factors are temperature and sun exposure rather than dietary requirements. D. bicanaliculata is lowly resistant to desiccation and thermal stress, therefore, inhibits lower intertidal zone areas (Mitchell, 1980). When understanding the encrusting species zonation trends, it is evident that abundance increases when moving down-shore. Due to sun and wave exposure, H. banksii predominantly inhabited the hormosira zone, absorbing sufficient sunlight for photosynthesis while remaining partially submerged in water to prevent desiccation (Lewis, Johnson & Wright, 2021). They are less abundant in the oyster zone because detached fragments move up-shore during high tide (McKenzie & Bellgrove, 2008), becoming trapped in the rockpools.The abundance of C. officinalis in the hormosira zone is due to its ability to thrive in wave-exposed conditions where sediment continuously flows, leading to accumulation within its dense turf structure, which prevents the settlement of competitor algae (Bussell, 2003).While C. columna was present across each zone, even coverage was shown amongst the oyster and hormosira zone. Though their shells combat high heat and wave energy environments, their location on the shore ensures they will not desiccate, as most barnacles can only withstand heat for short periods (Buckeridge & Reeves, 2009). The outliers appear to be Midge larvae and X. parietina, appearing only within the barnacle [Midge larvae] and oyster zone, excusing the <0.2% of coverage in the periwinkle zone [X. parietina]. Though X. parietina settles on near-shore rocks and trees, their spores are likely dispersed by wind, trapping the species in the rockpools and crevices of the oyster zone. This species shows higher germination rates when water salinity is at 0% (Hinds, 1995), indicating why there is a limited abundance in each zone.Though Midge larvae have adapted to withstand various conditions (Eldridge, 1992), North Head’s rock pools provide the most suitable habitat, providing shelter and constant water residence. When speaking about increased diversity near the low tide mark, each zone provided appropriate habitats for the observed species, whether the location depended on food, shelter, or wave/sun exposure. Due to its distance from the low tide mark, the periwinkle zone receives less water, so it is low in species diversity.Though the barnacle and oyster zone’s rock pools and crevices provide a constant aquatic habitat, over-exposure to the sun is still dangerous; subsequently, species with protective shells, such as barnacles, oysters, and snails, are adapted to live in these environments.The hormosira zone provides the most water exposure, explaining the range of species inhabiting this area. From the observed species, it was clear that some mobile individuals migrate down-shore to feed while encrusting species such as macroalgae required consistent water flow to avoid desiccation. Conclusion
This research sheds light on the abundance and diversity of intertidal species in four North Head zones. The results reveal a positive correlation between the distance from the low tide mark and species diversity. The study highlights the significance of various factors, such as food availability, shelter, wave and air exposure, and sun exposure, in determining the distribution and abundance of intertidal species. The observed trends in species diversity demonstrate how each zone offers suitable habitats that contribute to the success of the species. The zones located closer to the low tide mark experience more waves and less sun exposure, which appears to have contributed to the success of the recorded species. These findings support the hypothesis that species diversity increases as you get closer to the low tide mark.
References
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